1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be employed in measuring the concentration of a selected component of exhaust gasses emitted from automotive engines for use in air-fuel ratio control, and more particularly to an improved structure of such a gas sensor which is equipped with a built-in air ventilation filter and designed to ensure the stability of waterproofing and ventilation.
2. Background Art
There are known gas sensors to be installed in an exhaust pipe of an internal combustion engine for automotive vehicles to measure the concentration of oxygen (O2) contained in exhaust emissions. FIG. 18 shows such a type of a gas sensor 9.
The gas sensor 9 consists essentially of a sensor element 910 to measure the concentration of a gas (will also be referred to below as a measurement gas), a housing 911 in which the sensor element 910 is retained through a porcelain insulator 912, and an air cover 92 joined to a base end (i.e., an upper end, as viewed in the drawing) of the housing 911.
A rubber bush 93 is fitted in the base end of the air cover 92 to form a hermetic seal therein. The rubber bush 93 also retains therein leads 913 connecting electrically with the sensor element 910.
An outer cover 95 is fitted around a base end portion of the air cover 92. The joint between the outer cover 95 and the air cover 92 is achieved by pressing or crimping them radially inwardly to form an annular crimped portion 923.
A water-proof ventilation filter 94 is nipped firmly between the air cover 92 and the outer cover 95 by two annular crimped portions 950 which are away from each other in an axial direction of the gas sensor 9. The outer cover 95 has formed therein air intake openings 951 which face air intake holes 921 through the ventilation filter 94. Fresh air is introduced as a reference gas from the air intake openings 951 inside the gas sensor 9 through the ventilation filter 94 and the air intake holes 921.
However, the arrangement of the ventilation filter 94 between the air cover 92 and the outer cover 95 undesirably facilitates the transmission of thermal energy from the air cover 92 and/or the outer cover 95 to the ventilation filter 94. A long-term exposure of the gas sensor 9 to exhaust gas from the internal combustion engine will, therefore, result in thermal deterioration of the ventilation filter 94, which leads to reduction in waterproofing property of the ventilation filter 94 at the crimped portions 950, so that water may enter the gas sensor 9 at the air intake holes 921.
Japanese Patent First Publication No. 2001-235445 discloses a gas sensor 90, as illustrated in FIG. 19, which is equipped with a rubber bush 93 made up of two parts: an upper rubber block 93a and a lower rubber block 93b which have center air inlet holes 934 and hold a ventilation filter 94 firmly therebetween.
The gas sensor 90, however, has the following drawback.
The rubber bush 93 is pressed inwardly at all times by crimping the circumference of the air cover 92 to form a crimped portion 923. This may cause the air inlet holes 934 to deformed elastically, resulting in a lack in amount of air introduced into the gas sensor 90 through the center intake holes 934.